The present invention relates to a magnetic recording medium based on the longitudinal recording system, appropriate for ultra-high density recording, capable of obtaining thermally stable recording information with low noise.
It is desired that a magnetic disc device has a large capacity as an external magnetic recording device for a computer. For increasing the capacity, or achieving a higher recording density, the challenge for a magnetic recording medium is to lower noises. A magnetic recording medium has, for example a structure formed by providing a Co-alloy recording magnetic layer such as CoCrTa, CoCrPt and an overcoat film on a substrate via a Cr underlayer controlling the crystalline magnetic anisotropy of the magnetic layer (JP-A No. 257618/1987, JP-A No. 197018/1988), and in the past, the grain size was made finer, the film thickness was made thinner and the coercivity was made higher for lowering of noises. Thereby, the magnetization disorder in the magnetization transition region between recording bits, which might cause noises, can be smaller and the magnetization transition region can be narrower. However, finer magnetic grains and thinner recording layer might cause thermal fluctuation in the recorded magnetization and accordingly, the magnetization might be decayed. In general, the value Ku·V/(k·T) calculated by dividing the product of a magnetic anisotropy coefficient Ku and a grain volume V by the product of a Boltzmann's constant and a temperature T is known as thermal stability factor (IEEE Trans. Magn. 30 (1994) p. 4230). Ku·V/(k·T) indicates that a medium is thermally more unstable as this value gets smaller. From the thermal stability factor, even when the grain size is finer and the film thickness is thinner, thermal stability can be achieved by using a large Ku material. However, a medium with large Ku cannot be recorded by a magnetic head because large Ku is equivalent to a large anisotropic magnetic field. With the similar reason, the coercivity cannot easily be made larger.
As a means for achieving high recording density without increasing Ku, a magnetic recording medium having at least two ferromagnetic layers antiferromagnetically coupled with each other via a nonmagnetic layer was proposed (JP-A No. 148110/2001). At the Intermag international conference (Digest (Intermag 2000 conf.) IEEE, HT-01) held in April 2000, and in Appl. Phys. Lett. 77(2000) 3806, a medium formed by antiferromagnetically coupling two-layered and three-layered ferromagnetic layers (antiferromagnetically-coupled medium (‘AFC medium’ for short)) was reported to have excellent thermal stability and read-write characteristics. Then, these AFC media have been energetically investigated at various companies and institutions and have been used for today's magnetic disc devices.
FIG. 2 shows a simple schematic diagram illustrating a sectional structure and magnetic moments of a conventional magnetic recording medium when the AFC medium has two ferromagnetic layers. As shown in the drawing, the magnetic moment of the lower ferromagnetic layer 3 is in the opposite direction to the magnetic moment of the upper ferromagnetic layer 5. This is because the nonmagnetic layer 4 containing a material made of Ru, Cr, Rh, Ir, Cu or their alloys is used for antiferromagnetically coupling ferromagnetic layers 3, 5 placing the nonmagnetic layer therebetween. Where the saturation magnetization of the lower ferromagnetic layer 3 is MS1, the film thickness is t1, the saturation magnetization of the upper ferromagnetic layer 5 is Msu, and the film thickness is tu, the effective product (Mst)eff of the saturation magnetization and the film thickness of the entire recording layer is shown below.(Mst)eff=Msutu−Mslt1 
Accordingly, (Mst)eff of the recording medium is smaller than that of a single-layered medium made solely of a second ferromagnetic layer, and the noises would be reduced. In addition, since both the upper and lower ferromagnetic layers contribute to achieving thermal stability, the thermal stability would be improved compared to the single-layered ferromagnetic layer medium.
However, as the capacity gets larger, even with using the AFC medium, it would be difficult to achieve higher recording density. This is because finer magnetic grains and thinner films are increasingly required for obtaining required SNR (signal-to-noise ratio), and at the same time, the structure of the AFC medium for securing thermal stability has come to the limit. That is, for securing finer magnetic grains and thinner films and thermal stability at the same time, it is required to increase the thickness of lower ferromagnetic layers while increasing the thickness of upper ferromagnetic layers. However, the antiferromagnetic boundary coupling energy density Jex induced by the nonmagnetic layer is limited, and the magnetic coupling field Hex generated by Jex, which applies to the lower magnetic layer would be reduced in inverse proportion to the film thickness t1 of the lower ferromagnetic layer, as shown in the following formula.Hex=Jex/(Mslt1)
Accordingly, as the film thickness is larger, the antiferromagnetic coupling of the upper ferromagnetic layer and the lower ferromagnetic layer would be weaker and the effective film thickness would be increased instead of reducing the thickness. This means that differently from the initial intention, (Mst)eff would be larger, and the noises would be increased. Additionally, the thermal stability would be deteriorated.
In JP-T No. 515028/2004, it is disclosed that lower noises and improved thermal stability can be achieved by providing two upper ferromagnetic layers in an AFC medium and by increasing the saturation magnetization of the ferromagnetic layer further to a substrate than that of the other ferromagnetic layer. However, according to the inventors' investigation, it was found that good SNR can never be obtained even by increasing the saturation magnetization. Additionally, since increasing of (Mst)eff should be avoided, the ratio of saturation magnetization of the two layers is limited. That is, higher recording density is also limited.
In JP-A No. 85729/2003, it is disclosed that the layer to a substrate side can have a function for preventing turbulence in crystalline magnetic anisotropy at the layer to the substrate side, and good thermal activation and read-write characteristics (medium noise, etc.) can be achieved by providing a plurality of upper ferromagnetic layers in an AFC medium and in a case of providing, for example two layers, by providing one of the layers to the substrate side with thinner film thickness than that of the other layer. This means that the layer to the substrate side has works less as a recording layer.